Coating Bio-hybrid Pearls by Atomic Layer Deposition

Pearls are essentially a hybrid material of bio-organic and inorganic with micro-and nanostructured composition. The beautiful luster of a natural pearl is explained by nano-optics and the layered structure of the material. Pearls degrade under high temperature and low and high humidity environments and are susceptible to surface scratch without surface protection and chemicals such as weak acids. As a result, pearls require good handling, caring, and protection during wearing and storage. Currently, during the process of producing pearls jewelry, pearls are rubbed with a thin layer of wax or lacquer or organic coating to achieve such protections. However, due to pearls’ complicated surface structures, the coating is not able to cover or conform well enough with all the exposed surfaces or interfaces. As a result, such a coating process is not optimal. In this work, Atomic Layer Deposition (ALD) is used to coat a conformal coating of Al₂O₃. ALD offers a high conformality which conforms to structures with an even a 100:1 aspect ratio and down to atomic precision. Such coatings can be very conformal to the nanostructured surface of pearls and are also very robust and durable. Meanwhile, we can also utilize such coatings to introduce additional functionality such as adding color and certain optical functions. Our work showed that ALD coated surfaces of pearls significantly reduce surface roughness while improving the durability of pearls.